Записки программиста, обо всем и ни о чем. Но, наверное, больше профессионального.

2011-06-28

load average

Как-то я раньше не задумывался над природой пузомерки, которая загруженность процессора показывает. По виндовой привычке смотрел всегда на проценты загрузки камня и думал, что понимаю происходящее. А теперь я прочитал про показатель «load average» в Linux и прочих юниксах. Есть о чем подумать.

The state in question is CPU load—not to be confused with CPU percentage. In fact, it is precisely the CPU load that is measured, because load averages do not include any processes or threads waiting on I/O, networking, databases or anything else not demanding the CPU. It narrowly focuses on what is actively demanding CPU time.
...
The CPU percentage is the amount of a time interval (that is, the sampling interval) that the system's processes were found to be active on the CPU. If top reports that your program is taking 45% CPU, 45% of the samples taken by top found your process active on the CPU. The rest of the time your application was in a wait.
...
The load averages differ from CPU percentage in two significant ways: 1) load averages measure the trend in CPU utilization not only an instantaneous snapshot, as does percentage, and 2) load averages include all demand for the CPU not only how much was active at the time of measurement.
...
the load averages tell us by increasing duration whether our physical CPUs are over- or under-utilized. The point of perfect utilization, meaning that the CPUs are always busy and, yet, no process ever waits for one, is the average matching the number of CPUs. If there are four CPUs on a machine and the reported one-minute load average is 4.00, the machine has been utilizing its processors perfectly for the last 60 seconds.
...
The load-average calculation is best thought of as a moving average of processes in Linux's run queue marked running or uninterruptible. The words “thought of” were chosen for a reason: that is how the measurements are meant to be interpreted, but not exactly what happens behind the curtain.

linuxjournal.com/article/9001

Если проценты загрузки показывают долю времени (квантованного), в которое процесс пользует камень; а средняя загрузка показывает (условно) количество активных процессов, то как понять такое: на однопроцессорной машине, процентная загрузка = 3% а средняя загрузка от 3 до 4? И будет ли прок от пересадки системы на четырехядерную машину, при 3% загрузке одного камня?
Я думаю, так: приблизительно три процесса хотят чегой-то посчитать, но им приходится чего-то ждать. Но как снизить нагрузку — непонятно. Как узкое место найти?

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